The present invention relates to keyboards for business machines and more particularly to a typewriter having a low silhouette keyboard. The present invention keyboard includes keylevers that are arranged in multiple rows and suspended, biased, and guided transverse the keyboard by opposed cantilever mounted flexible support members at one end in cooperation with a single guide comb at its other end.
Typically, keyboards employed for use in business machines such as typewriters, include keylevers having a common pivot for the several rows of keys. It is desirable that the amount of angular keylever depression required for actuation of corresponding mechanism be consistent between rows. This requires that each row include a different design of the key mechanism for limiting key depression. A common solution for equalizing the dip of pivoting keylevers is for each row to vary the distance from the keylever stopping surface to its downstop limit.
It is also desirable in keyboard design that the reacting biasing force supplied for depression and restoration of the different keylevers of different rows be substantially equal. Pivoting keylevers have different lengths between the point of depression and its pivot for each row. The keylevers having the longest length have a greater mass and have a greater mechanical advantage than shorter keylevers and thus require different reacting forces applied thereto. Devices are often employed for equalizing the reacting force applied to the keylevers of different rows. Springs having various tensions were employed to bias the keys to their rest and restore positions to thereby equalize the force necessary to depress the different length keylevers. Very often the force required to depress the different keylevers varied substantially. Substantial variation of keylever depression force may result in non-acceptable print density of various printed characters. Mechanical means were often employed to adjust the degree of tension of such keylever spring devices to equalize the depressing force between the keylevers.
It is further desirable as a convenience for the operator, that keyboards include mechanism for adjusting the keytouch of all keylevers.
To overcome these deficiencies, attempts were made to improve the structure of the keylever and the biasing means attached thereto. One such example is U.S. Pat. No. 3,789,970 granted to George Costa, which discloses a keyboard having a plurality of keylevers disposed transverse the typewriter. The keylevers are biased to their rest and restore positions by two aligned rows of flexible members. The flexible members are disposed on a rigid U-shaped member in parallel spaced relationship. The keylevers are guided in their movement by dual guide combs at each end of the keylevers.
The Costa patent is an improved keyboard construction, in that it allowed for a substantially parallel depression, requiring substantially uniform reacting forces for depression and restoration among all keylevers with little friction during movement. The construction has a disadvantage in that an upper flexible member is tensioned, and a lower member is compressed during keylever depression. The lower compressed flexible member is capable of buckling which may result in an undesirable keylever motion. Such buckling further tends to reduce the life of the compressed flexible member. The construction has another disadvantage in that the keylevers were guided in guide combs located at both extremeties of the keylevers. This has a disadvantage of being a costly dual comb construction. The Costa patent further, does not disclose any mechanism for providing the desired adjustable keytouch feature. Additionally, the construction has a disadvantage by limiting this keyboard to a high silhouette.
The phrase "low silhouette" in this disclosure refers to a keyboard profile having minimal distance between key tops and the surface on which the machine rests. A low silhouette keyboard provides equal dip for all keylevers irrespective of row location and without having different key mechanism designs between the rows for limiting key depression. Additionally, since the path the keylever reciprocates is substantially perpendicular to the reacting biasing keylever force, and the mass of all keylevers is substantially the same, the necessary biasing force applied to all keylevers is substantially equal.
My U.S. Pat. No. 3,616,886 issued Nov. 2, 1971 discloses a low silhouette keyboard having equal dip keylevers. The construction of this keyboard includes a sagging four bar linkage pivotally connected on each keylever. When the keylever is depressed, a solenoid actuated bail moves a spring biased slide rearward into a ball interlock. There is a slide provided adjacently below each keylever. My present invention avoids the above design complexities which add to the manufacturers cost. The construction has another disadvantage, in that the compact profile is limited by having a portion of the slide located below the keylevers. A further disadvantage is that the keyboard does not include design for a keytouch adjustment mechanism.
Another example attempting to equalize the biasing keylever reacting force, is U.S. Pat. No. 3,974,905 issued Aug. 17, 1976 to Wendell C. Johnson. This patent discloses a unitary molded key mechanism having key arms with a single cantilever pivot point. The biasing key arm tension is equalized by varying the cross-sectional area of the key arms proportional to the length thereof. The key mechanism includes a support post formed at an angle onto which keys can be mounted, an actuating stem extending below the pivoting key arm for actuating mechanism. The key mechanism also includes a key arm guide and a stabilizing member that extends into guide slots of a guide plate.
The construction has a disadvantage by limiting this keyboard to a high silhouette, by having stems extending downward cooperating with actuating mechanism. Further, the key posts are formed at an angle to compensate for upward tilting of the key mechanism. This construction has another disadvantage of employing costly dual guide members. With one guide member at an end of the key arm and another below the key arm, the relationship of both must be properly aligned to permit the key arm to function properly without binding.